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Restoration of a Sandstone Facade: Fromthe Project to the MonitoringEmma Cantisani a , Daniele De Luca b , Piero Frediani a , CarloAlberto Garzonio b , Marilena Ricci b & Francesca Stori ca CNR Institute for the Conservation and Enhancement of CulturalHeritage, Florence, Italyb Department of Restoration and Conservation of ArchitecturalHeritage, University of Florence, Florence, Italyc Studio STORI Architettura, Florence, Italy

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To cite this article: Emma Cantisani, Daniele De Luca, Piero Frediani, Carlo Alberto Garzonio,Marilena Ricci & Francesca Stori (2012): Restoration of a Sandstone Facade: From the Project to theMonitoring, International Journal of Architectural Heritage, 6:3, 290-301

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International Journal of Architectural Heritage, 6: 290–301, 2012Copyright © Taylor & Francis Group, LLCISSN: 1558-3058 print / 1558-3066 onlineDOI: 10.1080/15583058.2010.529237

RESTORATION OF A SANDSTONE FACADE: FROMTHE PROJECT TO THE MONITORING

Emma Cantisani,1 Daniele De Luca,2 Piero Frediani,1

Carlo Alberto Garzonio,2 Marilena Ricci,2 and Francesca Stori31CNR Institute for the Conservation and Enhancement of Cultural Heritage,Florence, Italy2Department of Restoration and Conservation of Architectural Heritage, Universityof Florence, Florence, Italy3Studio STORI Architettura, Florence, Italy

This work describes the restoration of a facade of an historical building, constructedfrom typical local sandstone, characterized by different subsequent decay phenomena andsubjected in the past to a conservation treatment. The sandstone was characterized, thequarry of provenance was identified, and the causes of the different decay phenomenawere investigated. Potential commercial products for the restoration were tested first in thelaboratory, then in situ selected products were applied and the effectiveness of treatmentwas subsequently monitored. Analytical methodologies such as x-ray diffraction (XRD),observation of thin sections under polarized microscope (OM), Fourier transform infraredspectroscopy (FT-IR), micro-Raman spectroscopy, and water absorption tests were employedin the laboratory, while nondestructive tests, performed with portable instrumentation suchas colorimeter and sponge contact, were carried out in situ.

KEY WORDS: sandstone, facade, decay, restoration, monitoring

1. INTRODUCTION

Cortona (Arezzo, Italy) is one of the most beautiful towns in Tuscany. Foundedby the Etruscans, Cortona is a proud example of Medieval and Renaissance architecture.Historical palaces, constructed using local sandstones, are present on its main roads andsquares (Palazzo Comunale, 12th–16th centuries; Palazzo Quintani, 12th century; PalazzoTommasi Fierli, 15th century; Palazzo Venuti, 16th century).

Located on Via Guelfa in Cortona is the palace called “Cristofanello” (Figure 1),from its designer, originally a Laparelli property, then linked to Mancini Sernini andtoday the office of the Banca Popolare di Cortona. Designed and completed in the1533 from Battista di Cristofanello of Bartolomeo di Senso (for others Giovanni Battistaof Cristofano Infregliati, called Cristofanello), it was built using local sandstone (Mancini1897). This stone has been subject to different decay phenomena, strictly connected with itsmineralogic and petrographic properties. In the past, fragments of the facade fell and someconservation took place and was documented. The most important conservation treatment

Received 28 June, 2010; accepted 29 September 2010.Address correspondence to Emma Cantisani, CNR Institute for the Conservation and Enhancement of

Cultural Heritage, via Madonna del Piano 10, Sesto Fiorentino, Florence, Italy. E-mail: [email protected]

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SANDSTONE FACADE RESTORATION 291

Figure 1. The facade of Cristofanello Palace (color figure available online).

was performed in 1974. At that time, the detachment of stone and the loss of consistencyof the building masonry not only obligated a restoration of the facade but also required astatic consolidation. Some documents of that period report the consolidation of the buildingto its full height. This consolidation of the facade was very complex: less stable portions,such as columns and angular frames, were replaced; tiling was carried out on architecturalelements characterized by the loss of shape, and a sealing of fissures, termed mantelline,were placed to guarantee the outflow of water. A consolidation product as well as a water-proofing treatment was applied on the facade, and, in order to bind some partially detachedportions of stone, many holes were drilled to insert adhesive resins (Figure 2). This opera-tion, very coherent for that time, was not able to prevent the subsequent decay; in fact, thedetachment of the material with the fall of fragments started again.

In the spring 2006, after a portion of molding fell, the Department of Restoration andConservation of Architectural Heritage of Florence University commenced a work plan toevaluate the state of conservation of this facade and to determine a procedure of restoration.

INTERNATIONAL JOURNAL OF ARCHITECTURAL HERITAGE 6(3): 290–301

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292 E. CANTISANI ET AL.

Figure 2. The state of conservation of a portion of facade in 1974.

Initially a conservation status map of the building was prepared. This map confirmed thatthe conservation carried out in 1974 did not stop ongoing decay from penetrating waterinto the material, causing a loss of consistency of the building stone while the facade’sform became almost indiscernible.

In the lower level of the facade black crusts were prevalent, especially in the area pro-tected by percolating waters; under some crusts granular disintegration was also present.Exfoliation phenomena, detachment of portion of material and swelling were evident atthe first floor level. A total loss of molding and of working traces was registered in someareas.

Other very dangerous decay phenomena were related to the presence of microc-racking and to the detachment of the surfaces that can cause the fall of entire portionsof blocks.

The frieze located above the second ledge became virtually unreadable, only somecharacters were clearly recognizable. At the top level decohesion, arenitization, and loss ofmaterial were recurrent phenomena, especially under the roof. At this level it was evidentthat in the 1974 intervention some columns have been replaced with new columns realizedusing similar sandstones, which remain in a good state of conservation.

Some decay phenomena are shown in the Figures 3a–d.

2. SAMPLING AND ANALYTICAL METHODOLOGIES

Sampling was carried out in order to characterize the sandstone and to evaluate itsstate of conservation. In Table 1 analyses of these samples are provided, together with thesampling location and the analytical methodologies used. Thin sections (30 μm thickness)were prepared and observed under polarized light, using a Zeiss microscope, under differ-ent magnifications. Powders were analyzed with a Philips model PW 1729 diffractometerwith radiation CuKα1= 1,545Å, goniometer speed 2◦/min, angular range 3◦<2θ<60◦.


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(a) (b)

(c) (d)

Figure 3. Different decay phenomena at variable scales: erosion, scaling (a), decohesion and dissolution (b),flaking and scaling (c), swelling (d) (color figure available online).

The main mineralogical composition and clay minerals composition were deter-mined because these data are very useful for the assessment of outcrops (Cipriani 1958a;1958b). Furthermore numerous studies have documented the role of the clay minerals asso-ciation in the assessment of the sandstone’s provenance (Banchelli et al. 1997; Fratini et al.1998; 2002). The chemical analyses of the powders were performed using Fourier trans-form infrared spectroscopy (FT-IR) (a Perkin Elmer Spectrum 100 spectrophotometer withthe attenuated total reflectance (ATR) accessory [Perkin Elmer, USA]) and a micro-Ramansystem (Renishaw RM2000 single grating[1200 g/mm] spectrometer, diode array 782 nm[Rainshaw, United Kingdom]) for the identification of organic and inorganic compounds.

From an irregular block of 10 × 5 × 2 cm fallen from the facade in the spring of2006, small samples were obtained for the physical characterization of the material. Wateraccessible porosity was measured using a hydrostatic balance (NORMA ISO 6783, 1982).Such value was obtained on samples kept at 60◦ C up to constant weight, until the saturationstabilized under room pressure. Archimedes Law was applied, where Ps is the dry weight,Pb is the wet weight at the saturation point, Pi is the hydrostatic weight measured by thebalance, Va is the volume of samples, Vw is the volume of water inside the pores, γ w is thedensity of water at the temperature of the measure.

Vw = (Pb − Ps)/γw (EQ1)


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Table 1. Names of analyzed samples, sampling location, and the used analytical methodologies

Sample Description Analytical methodologies

SC1 Fragment, first floor, gray block Optical microscope (OM), x-ray diffraction (XRD)SC2 Fragment, first floor, brown block OM, XRDSC3 Fragment, second floor, brown block OM, XRDSC4 Gray crust, second floor OM, XRDSC5 Gray fragment, second floor OM, XRDSC6 Gray fragment on pillar OM, XRDSC7 Fragment, second floor, last pillar OM, XRDSC8 Fragment OM, XRDSC9 Fragment under frieze OM, XRDSC10 Black crust, first floor XRD, Fourier transform infrared spectroscopy (FTIR),

micro-Raman spectroscopy (micro-Raman)SC11 Black crust on the sandstone, first floor XRD, FTIR, micro-RamanPO1 Powder, first floor, grey block (cf. SC1) XRD, FTIRPO2 Powder, first floor, brown block (cf. SC2) XRD, FTIRPO3 Powder block above SC1 e SC2. XRD, FTIRPO4 Powder above brown block (cf. SC3) XRD, FTIRPO5 Powder gray crust (cf. SC4) XRD, FTIR, micro-RamanPO6 Powder swelling XRD, FTIR, micro-RamanPO7 Powder brown block XRD, FTIRPO8 Powder third ashlar XRD, FTIRPO9 Powder in the arenitized area XRD, FTIRPO10 Black crust XRD, FTIRPO11 Powder swelling XRD, FTIR, micro-RamanPO12 Black powder, basis ashlar XRD, FTIR, micro-Raman

Va = (Pb − Pi)/γw (EQ2)

Where Vw/Va % is the porosity accessible to the water.The requirement of portions of material having the same characteristics of the stone

used in the facade addressed the issue of determining the historic or modern quarries orotherwise quarries with sandstones having characteristics very close to those utilized inthe facade of the palace. On the basis of historical and geological information two dif-ferent quarries (named Q1 and Q2) located near to the town of Cortona, were identified.Mineralogic and petrographic analyses over the samples of material collected from themwere carried out.

The thin sections were observed under the optical microscope (OM) and the powderswere analyzed by X-ray diffraction (XRD) for the identification of the main mineralogi-cal and clay minerals composition. From the material having characteristics very closeto the sandstone used in Palazzo Cristofanello, samples of 5 × 5 × 2 cm and 2 × 2× 2 cm were collected to evaluate the water absorption using a hydrostatic balance andthe capillarity tests (NORMA UNI 11859, 2000). The effectiveness of some commercialproducts to reduce the water porosity was tested:

ethyl silicate (ETS, solution in white spirit), that in the scientific literature is reportedas a product diffusely used for the restoration of sandstones (Snethlage 2002); organosilox-ane oligomers containing small amount of tri-functional monomers, formulated in 10%dearomatized mineral solvent, as a water repellent (called SR). Upon application on the


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stone, the product reacts with the water present in the atmosphere giving silanol groups thatreact together resulting in the formation of three-dimensional-crosslinked polysiloxanes.

The effectiveness of treatments was defined as:

E.P.% = (A1 − A2)/A1 ∗ 100 (EQ3)

where A1 is the amount of water absorbed in the capillarity tests before treatment and A2 isthe amount of water absorbed after treatment.

The application of ETS was carried out with a brush, until the sandstone was satu-rated. The amount of absorbed product was evaluated by weighing the samples before andafter the treatment until reaching a constant dry weight. Six months after this treatmentwith ETS, the samples were treated with the previous described siloxane resin using brush.The tests of water absorption through capillarity have been carried out in order to evaluatethe waterproofing effect.

The products selected in laboratory: ETS as consolidant, and siloxane resin (SR)as protective, were applied on the facade. Due to the absence of nondestructive meth-ods for the assessment of the effectiveness in situ of consolidative treatments prompts usto employ only methods for the assessment of the effectiveness of protective treatment.The in situ water absorptions were performed using the contact sponge tests as suggestedin the methodology described by Tiano and Pardini (2004). Colorimetric tests were alsoperformed on the same areas using a Minolta colorimeter.

The restoration project has been completed in the autumn of 2008, but it hasbeen followed by an ongoing monitoring plan that has included two control inspectionsin the summer of 2009 (6 months after the treatment) and in the winter 2009/2010(14 months after the treatment) using colorimetric and water absorption tests. This casestudy focuses on the research that identifies the consolidative effectiveness tests to beperformed in situ.

3. RESULTS AND DISCUSSION

3.1. Palace Samples

Optical microscope (OM) analyses allow the identification of the stone used for thefacade as a sandstone constituted by a framework of fine grained quartz (mono and poly-crystals), Kfeldspars, plagioclases, muscovite, biotite and metamorphic rock fragments.The matrix is predominantly clay, rarely calcite, and in the samples from the superficialportion ashlars is limited to a small amount. The plagioclase shows naturally alterationphenomena (sericitization). At microscopic level the incipient arenitization of the rock isevident, the clay matrix was almost all washed out (Figures 4a, 4b).

It is known that the decay of sandstones is specifically connected with themineralogic, petrographic, and physical characteristics of material. The presence of waterin the masonry determines the dissolution of calcium carbonate that subsequently, in theevaporation stage, reprecipitates on the surface, forming a less porous crust causing reduc-tion of the permeability of the stone and affecting the detachment of material. Anothercause of decay is linked to the presence of clay minerals in the matrix, causing the deco-hesion of material. The knowledge of the intrinsic characteristics of material and theextrinsic environmental conditions (presence of water, its circulation, its sources, presenceof pollutants) is essential for planning a conservation intervention.


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(a) (b)

Figure 4. Thin section of a sample in which the detachment phenomenon is evident (polarized microscope:magnification 6,25X, n // [a], n ⊥ [b]) (color figure available online).

Figure 5. Micro-Raman spectra of a black crust (C= amorphous carbon).

The integrate FT-IR and micro-Raman analyses allow determination of the compo-sition of powders and to exclude the presence of any treatment product, that over a span of30 years disappeared at depth in the stone (Figures 5 and 6). The crusts were formed bygypsum and amorphous carbon. The physical tests performed with hydrostatic balance inthe samples obtained from the fallen block, show a porosity accessible to water of 6.1%(±0.8) and a water imbibitions coefficient (in weight) of 2.4% (±0.3).

3.2. Identification of an Efficient Treatment

In order to evaluate the commercial products for the restoration, a large quantityof material was required. Research focused on the identification of the original sourcequarry or quarries with sandstones very close to those employed at the palace. On the basisof historical information and geological knowledge of the area, two possible abandonedquarries were identified. These quarries are located near Cortona, the first in proximity ofthe Eremo le Celle (Q1), the second in the locality named Torreone (Q2). In these areas


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Figure 6. FT-IR spectra of a sandstone with a black crust (G=gypsum). The spectra reveal the presence ofsilicatic phases linked to the composition of sandstone.

outcrops of the Poggio Belvedere member of Macigno Formation were characterized byan alternation of thin/medium fine-grained and thick coarse-grained turbiditic strata (Plesiet al. 2002).

The sandstones collected in both quarries were analyzed using OM and XRD analy-ses. In Table 2, the results concerning the main mineralogical composition are reported. InTable 3, the composition and percentage of clay minerals are shown.

Samples data from both quarries are similar to those of the Palace, but the stonepresent in the quarry Q1 is closer also to the petrographic analyses in thin section (meangrain size, grain size distribution, sorting).

Samples of Q1 quarry were selected and some physical tests were performed. Thewater porosity was 5.5% (±0.2), while the weight imbibition coefficient was 2.20%(±0.08). These values are lower than those obtained on palace samples, but are, generally,almost high.

Laboratory blocks of 5×5×2 cm coming from the selected quarry Q1 were treatedwith commercial products (ethyl silicate and a siloxane oligomer). The effectiveness of

Table 2. Main mineralogical composition

SampleQuartz,

%K Feldspar,

%Plagioclase,

%Calcite,

%Clay +

accessory, %

Q1 35 9 15 4 37Q2 27 9 13 5 46Palace 25 9 10 6 50

Table 3. Clay minerals composition

Sample Kaolinite, % Illite, % Chlorite, % Vermiculite, %

Q1 35 30 20 15Q2 35 30 15 20Palace 35 30 20 15


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Table 4. Sandstones treated with commercial products: protective effectiveness

ProductApplied

product L/m2 Time, 2 months

Time, 6months∗ and 7

months∗∗

Ethyl silicate (ETS) 0.284 78 ± 3 89 ± 2∗Siloxane resin (SR) 0.724 — 92 ± 3∗∗

Figure 7. L∗, a∗, b∗ values of test samples before treatment and after treatment with ethyl silicates and siloxanicresin.

treatments has been evaluated through capillarity tests. In Table 4, the effectiveness oftreatment evaluated after 2 and 6 months from the treatment with ETS is reported.

After 6 months, all the samples have been treated with a SR and the test of waterabsorption has been repeated, after 1 month, showing a further decrease of the waterabsorption. These data confirm the suitability of these products for the treatment ofsandstones.

The change of color was tested after the application of the two treatments.In Figure 7, the values of L∗, a∗ and b∗ are reported, before treatment (BF) and after treat-ments with ETS and SR. The �E values are, respectively, 1.92 after ETS treatment, and1.46 after the SR treatment.

3.3. Restoration of the Palace

The first stage of restoration at the Palace was the realization of a new system of roofgutters to avoid the presence on the facade of water percolating via the roof, because thepresence of water is considered to be the main cause of the physical-mechanical decay.


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Table 5. Water absorption through sponge contact tests

AreaBefore treatmentmg/(cm2 min)

After treatment with ethylsilicate (ETS) mg/(cm2

min)

After treatment withsiloxanic resin (SR)

mg/(cm2 min)

1 20 3 12 30 2 13 10 8 3

(a) (b)

Figure 8a, b. Details of the Palace after restoration (color figure available online).

The facade was then preconsolidated and the black crusts removed before treatmentwith selected commercial products. The preconsolidation was realized with ETS orinjections of lime fluid mortar in the partially detached portions. The totally detachedfragments were reattached with epoxy resins.

The black crusts were removed using a cellulose paste containing ammonium car-bonate. In a second step, a biocide product was applied with a brush to remove thebiological agents. The ancient plastering made of cement, iron springs and nails wereremoved, all the flows and micro-crackings were filled with a mortar composed of limeand stone powder. Some areas were selected to evaluate the conservation treatment on thebasis of their accessibility after the removal of scaffolds.

The data on the water absorption of the facade before and after the treatment with theselected commercial products are reported in Table 5. Some details of the restored facadeare showed in Figures 8a and 8b. In order to verify the effectiveness of the treatment overthe course of time, a monitoring schedule was instituted with the aim of establishing pre-ventive maintenance, and so avoiding emergency interventions. The data collected duringthe first 14 months of monitoring of the water absorption of the facade, realized in fourselected areas are reported in Table 6, while in Figure 9 the changes of L∗, a∗, b∗ valuesare reported. No significant variations of colorimetric parameters are shown (in each case�E < 3).

4. CONCLUSIONS

The facade of Cristofanello Palace, completed in the 1533, restored in the 1974, wassubject over time to subsequent decay phenomena. In 2008 a restoration intervention wasnecessary, after experimental research in the laboratory and in situ. This study emphasizes


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Table 6. Water absorption monitoring with sponge contact on selected areas

Area

Untreated area(reference)

mg/(cm2 min)

Area treated after6 months

mg/(cm2 min)

Area treated after14 months

mg/(cm2 min)

1 20 5 (RH 24%) 5 (RH 35%)2 30 10 (RH 39%) 4 (RH 47%)3 20 4 (RH 37%) 1.2 (RH 51%)4 10 2.6 (RH 43%) 2 (RH 51%)

RH = relative humidity.

Figure 9. L∗, a∗, b∗ values in the four areas detected after 6 months (May 2009) and 14 months (January2010) from the treatment.

that the restoration of the facade of an historical building requires the collaboration ofdifferent skills in order to identify the building stone sources, the causes of decay, and thebest commercial products appropriate for the consolidation and the protection as well asthe need of ongoing monitoring. This approach will review the effectiveness of treatmentsin order to avoid future emergency interventions.

The analyses confirm that local sandstone poorly cemented, with high capacity ofwater absorption, was used and the main decay phenomena were linked to the swellingof the clayey matrix. As a consequence some portions of the facade are subject to strongdecohesion phenomena caused by the arenitization of sandstone. The applied commer-cial products (ETS and siloxane resin) show a good efficiency in the reduction of waterabsorption (the protective effectiveness evaluated in the laboratory after 7 months is 92%).Good colorimetric stability and maintenance of hydro repellence were also observed onthe facade after 14 months of natural ageing.


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ACKNOWLEDGEMENT

The authors thank Banca Popolare di Cortona for the financial supports needed toconduct this research. Piero Frediani thanks projects TeCon@BC CP 57046 and ST@RTfor financial supports.

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Banchelli, A., F. Fratini, G., Germani, P. Malesani, and C. Manganelli del Fa. 1997. The sandstoneof Florentine historic buildings: individuation of the marker and determination of the suppliesquarries of the rocks used in some Florentine monuments. Science and Technology for CulturalHeritage 6(1):13–22.

Cipriani, C. 1958a. Ricerche sui minerali costituenti le arenarie: Sulla composizione mineralogicadella frazione sabbiosa di alcune arenarie Macigno. Atti della Società Toscana di Scienze Naturali65: 165–220.

Cipriani, C. 1958b. Ricerche sui minerali costituenti le arenarie: Sulla composizione mineralogicadella frazione argillosa di alcune arenarie Macigno. Atti della Società Toscana di Scienze Naturali65: 86–106.

Fratini, F., Manganelli del Fa, C., Baldini, L. 1998. The association of clay minerals and the prove-nance of the arenaceous building stones of Florentine architecture. In Proceedings of ASMOSIA 5.Interdisciplinary studies on ancient stone, eds. J. J. Herrmann, N. Herz, and R. Newman.June 11–15, 1998, Boston, MA. London: Archetype Publication Ltd. pp. 110–197.

Fratini, F., C. Manganelli Del Fa, E. Pecchioni, and S. Rescic. 2002. Clay mineral associations insandstone of Arezzo (Italy): A reliable tool for discrimination in architecture. In Proceedings ofASMOSIA 6. Interdisciplinary studies on ancient stone, ed. L. Lazzarini. June 15–18, 2000, Venice,Italy. Padova Switzerland: Bottega di Erasmo Aldo Ausilio Editore. pp. 193–197.

NORMA ISO. 1982. Coarse aggregates for concrete: Determination of particle density and waterabsorption-hydrostatic balance method. NORMA ISO 6783.

NORMA UNI. 2000. Assorbimento d’acqua per capillarità–Coefficiente di assorbimento capillare.NORMA UNI 11859, Milano, Italy.

Mancini, G. 1897. Cortona nel medioevo. Firenze, Italy: Multigrafica Ed.Plesi, G., L. Luchetti, A. Boscherini, et al. 2002. The Tuscan successions of the high Tiber Valley

(foglio 289: Città di Castello): Biostratigraphic, petrographic, and structural features, regionalcorrelations. Bollettino Società Geologica Italiana, 425–436.

Snethlage, R. 2002. Silicon organic compounds for natural stone conservation. New products and lab-oratory tests. In Proceedings of the International Congress: Silicates in Conservative Treatments.Torino, Italy: February 13–15, 2002. LOG ED, Genova, 27–36.

Tiano, P., and C. Pardini. 2004. Valutazione in situ dei trattamenti protettivi per il materiale lapideo,proposta di una nuova semplice metodologia. ARKOS 5: 30–36.


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